1. Field of the Invention
The present invention relates to an image forming apparatus to which electrophotography is applied and which uses a plurality of developing devices by switching one to another.
2. Description of the Related Art
Generally, image forming apparatuses to which electrophotography is applied comprise a photosensitive member, a charging device, an exposing device, a developing device, a transfer device and a fixing Device. Especially, a multi-color image forming apparatus for forming a multi-color image comprises a plurality of developing devices. The developing devices have toner containers, respectively, which store toners of different colors, to thereby form multi-color images. As the developing devices are switched from one to another in sequence, toner images that correspond to the color of the toner which is contained in respective toner containers, are formed on the surface of the photosensitive member. Since the toner images are laid, one upon another, a multi-color image is formed on the surface of a recording sheet.
There is a type of a multi-color image forming apparatus comprising a developing device assembly that has developing devices bundled together. The multi-color image forming apparatus switches one developing device to another by making the developing device assembly rotate like the cylinder of a revolver. The multi-color image forming apparatus repeats the image forming routine, each time carrying out charging, exposing, developing and transferring for the photosensitive member. Thus, the apparatus forms a multi-color image on the surface of a recording sheet. The multi-color image forming apparatus needs a longer time to form a complete multi-color image than a single color image forming apparatus needs to form an image, because it repeats the image forming routine as many times as sorts of color it uses.
Recently, as the demand for multi-color image forming apparatuses has increased, users now request multi-color image forming apparatuses that can form multi-color images quickly. There is one of the known techniques for forming multi-color images quickly, which raise the rotational speed at which the developing device assembly rotates to switch one developing device to another quickly. In this method, the inertia of the developing device assembly affects the rotation of the assembly, at the start and stop of rotation. A drive unit that rotates at high speed and has a large torque must be used to rotate the developing device assembly, against the inertia. In the multi-color image forming apparatus, each developing device of the developing device assembly, that is rotating, must be stopped at a predetermined position, with respect to the photosensitive member.
There is a method that is available for reducing the influence of inertia. In this method, reducing the storage capacity of each toner container decreases the weight of each developing device. However it is uneconomical, because the toner container must be replaced very often, when the storage capacity of the toner container is decreased.
The capacity of the drive device and the storage capacity of the toner containers therefore limit the increase in the rotation speed of the developing device assembly. There is a limit for the image forming speed of an image forming apparatus that switches and uses a plurality of developing devices.
An image forming apparatus according to the present invention is designed to form images at an increased speed.
The image forming apparatus according to one embodiment of the present invention includes a plurality of developing devices, a plurality of developing device assemblies, and a controller. At least one developing device is provided for each of toners of different sorts, prepared to form an image. Each of the developing device assemblies has a plurality of developing devices. While the developing devices of one developing device assembly are operating, the controller switches developing devices of another developing device assembly, from one to another.
An image forming apparatus according to another embodiment of the invention includes a plurality of developing devices, a first developing device assembly, a second developing device assembly and a controller. At least one developing device is provided for each of the toners of different colors, prepared to form an image. The first developing device assembly has a plurality of developing devices containing toner of at least one of a plurality of colors. When the first developing device assembly is rotated, its developing devices are switched, from one to another. The second developing device assembly has a plurality of developing devices containing toners other than the toners contained in the developing devices of the first developing device assembly. When the second developing device assembly is rotated, its developing devices are switched, from one to another. While any one of the developing devices of the first developing device assembly is being driven for developing operation, the controller switches the developing device of the second developing device assembly, from one to another. Conversely, while any one of the developing devices of the second developing device assembly is being driven for developing operation, the controller switches the developing device of the first developing device assembly, from one to another.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.